Measurement of the magnetic field of moving vehicles is known. If vehicles always moved at a single speed, the signals could be correlated directly. Since vehicles change speeds and do so unpredictably, the form may be stretched or compressed or distorted into regions of variable stretching and/or compression. Some parts of the signal remain repeatable. The industry convention is to hit on the simplest method. A single component, most commonly the z component, is selected for consideration. Maxima and minima are detected in the data stream, and are listed in order min[1], max[1], min[2], max[2], min[3], max[3], and so on. These values are directly correlated.
Problems with the conventional method include throwing out almost all information aside from extrema for an arbitrary field coordinate right at the outset; magnetic fields are treated as disjoint measurements with all spatial and time-evolution theory discarded entirely; and the statistics of maxima and minima vary significantly amongst vehicles, with small numbers of extrema often dominated by leading and trailing extrema. Sensible and repeatable interpretation of respective statistics suffers severe limitations.